Indoor Airflow Analysis of Duplex and Single Apartment within Shanghai Building

Camille Alloca

    A CFD analysis was performed on indoor airflow within a duplex and single apartment in Shanghai, China, with the intent of maximizing natural ventilation through the building.  To perform the indoor airflow analysis, architectural floor plans and unit layouts, as well as window placement and size, were provided for each apartment.  The assumptions made and results reported for this analysis are outlined in this paper.

>INDOOR ENVIRONMENT SETUP

     In the CFD simulation of the indoor environment, the following parameters were solved for and analyzed: pressure (p), velocity (v), temperature (T), CO­₂ concentration (c), humidity ratio (w), relative humidity (f), percentage of persons dissatisfied (PPD), persons dissatisfied due to draft (PD), ventilation effectiveness (h), and age of air (t).  In order to solve these parameters, input values had to be chosen based on reasonable conditions.  These input parameters, chosen for both the duplex and single apartment CFD simulation, are shown in Table 1.  The CFD simulation for both apartments used the Renormalization group (RNG) k-e turbulence model.

Table 1: Input parameters for CFD simulation of duplex and single apartments

     *  Both inlet and outlet pressures were input and vary between the duplex and single apartment.

      **Values refer to each of the 4 occupants.

DUPLEX APARTMENT

SINGLE APARTMENT

   The floor plan of the single apartment chosen for this analysis is shown in Figure 1.  The apartment consists of an open area, kitchen, bathroom, and three bedrooms (8 m x 14.8 m x 2.8 m).  The layout set up in the CFD program, PHOENICS, is shown in Figure 2.  In order to model the physical environment of this apartment, certain assumptions had to be made within the CFD program.  A southeast section of the apartment was not part of the apartment, and was therefore modeled as a blockage (see Figure 1).  Minimum furnishings within the apartment were modeled by placing a bed and closet in each bedroom, and a table in the living room.  This was done in order to get a rough idea of how blockages within the apartment effected the airflow patterns.  A 49 x 82 x 17 grid was set up, providing a fine mesh.

Figure 1: Single apartment layout

Figure 2: Single apartment modeled in CFD

     The single apartment is fully exposed on the south, east, and north side and partially exposed at the northern corner of the west side, as shown in Figure 2.  Given this exposure, an optimum window layout design was chosen.  Since the apartment layout consisted of many partitions parallel to the south facade (where the wind was directed), it was necessary to maximize the openings on all facades to obtain good cross ventilation.  Therefore, either one or two windows were placed in each room (see Figure 1).  Each window was assumed to be 1m x 1m, but was simulated as being open halfway, or 0.5 m wide x 1 m high.

     Windows were modeled as fixed-pressure outlets.  Pressures for each side of the building were obtained from the outdoor airflow analysis around a sealed building.  This outdoor analysis assumed a velocity of 2 m/s approaching the building site from the south.  For the indoor analysis discussed in this paper, outdoor pressures were taken at a height in the center of the building, or 18.2 m.  The pressures obtained from this analysis are shown in Table 2 for each window.  There was an uncertainty inherent in the exact location of extracting these outdoor pressures from the outdoor flow simulation that must be considered.

Table 2: Outdoor pressures at windows for z=18.2 m

Figure 3: (a) Velocity vectors and (b) contours at z=1.5 m

     For the single apartment, the airflow enters through both south-facing windows and two east-facing windows, as shown in Figure 3a.  This airflow created high ventilation rates through the hallway and through the entrance to the north bedroom and kitchen, where the air then exited the building at high velocities.  The air change rate at a building height of 18.2 m was calculated as

This air change rate allows for good natural ventilation through the building.  The design was therefore found to be satisfactory.  However, areas of low velocity in the center and south bedroom, ranging from 0.01 to 0.1 m/s, may not provide sufficient natural cooling.  There are also areas of extremely high air velocities through the hallway, north bedroom and kitchen, which may contribute to discomfort due to draft, as will be discussed later.  These areas of extreme low and high airflow within the single apartment still need to be addressed further.

     The temperature values were mostly constant throughout the single apartment.  Values for temperature were found to be about 24 °C in most of the space, with temperature reaching only 25 °C in those areas where heat sources were located.

Figure 4: Temperature contours at z=1.5 m

     The thermal comfort parameters calculated in the CFD simulation were percentage of persons dissatisfied (PPD), persons dissatisfied due to draft (PD), relative humidity (f), ventilation effectiveness (h), and age of air (t).  The PPD calculated for the single apartment is shown in Figure 5.  For most of the space in the apartment, a PPD was calculated between 5-6%.  However, there were areas within the apartment that reached PPD values of up to 30% and should therefore be of concern. A similar trend to that of PPD was found when calculating PD, except the fact that PD spanned over a larger range of values, from 0 – 100% (Figure 6).  Areas of greatest concern are those with high airflows: the hallway and entrance to the north bedroom and kitchen.  These locations contain PD values ranging from 40% to 100%. 

     The relative humidity (f) was calculated as a function of the temperature and humidity ratio (w) at a particular location.  Therefore, inlet and source humidity ratio values were input into the CFD simulation in order to calculate the relative humidity.  The range of values for f were found to be between 70 – 74% (Figure 7).  The ventilation effectiveness (h) was then calculated as a function of the inlet, outlet, and source concentration (c).  Therefore, inlet and source CO₂ concentration values were input into the CFD simulation in order to calculate h (see Table 1).  The range of values for h was found to be between 20-100% (Figure 8).  However, most of the space was characterized by h=100%.  Finally, the age of air (t) was calculated by assuming a unit source at every location in the room and t=0 at the inlets.  This produced results for t ranging from 0 – 102 s (Figure 9).  The age of air for complete mixing was determined to be:

which is within the calculated range for t.  The CFD calculation for h and t seems to show good mixing, without any areas of stagnant air.

     An overview of the range of values found for the various parameters solved is shown in Table 3.

Figure 5: PD contours at z=1.5 m

Figure 6: PPD contours at z=1.5 m

Figure 7: Relative humidity contours at z=1.5 m

Figure 8: Ventilation effectiveness contours at z=1.5 m

Figure 9: Age of air contours at z=1.5 m

Table 3: Solved parameters for CFD simulation of single apartment

In order to determine the effect of pressure changes along the height of the building on indoor airflow rates, CFD simulations were run at 3 m intervals in height for both the duplex and single apartment.  By extracting data from the outdoor flow simulation, outdoor pressure was plotted as a function of height at each of the openings.  At each interval in height, the appropriate outdoor pressure value was extracted for each opening and the CFD simulation was run in order to determine the ACH. 

For the single apartment calculation, outdoor pressures were extracted at each of the 8 openings as a function of height.  After running the CFD simulation at 6 vertical locations, the ACH in the single apartment was determined as a function of height, as shown in Figure 10.  This analysis allows for interpolation of airflow rates in the single apartment at all 12 stories of the building.

Figure 10: ACH versus height for the single apartment

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